A sintered Nd—Fe—B magnet has superior magnetic capabilities, and therefore has been widely applied in the fields of wind power generation, nuclear magnetic resonance, automobiles, computers, aerospace, household appliances and others, resulting in too much consumption of the main raw material Nd of the sintered Nd—Fe—B magnet. Since there is a large amount of Ho, Ho is selected to partially replace the metallic Nd in the magnet, which has great significance for comprehensive utilization of rare-earth resources. Also, since coercivity and temperature stability of the Nd—Fe—B magnet can be significantly improved with Ho, when the low-cost Ho, which is easily acquired in industrial production, is selected to partially replace the metallic Nd in the magnet, comprehensive production cost of the rare-earth magnet with high capabilities can be reduced.
It is described in “Effects of Adding Gd or Ho on Structure and Performance of Sintered Nd—Fe—B Magnet” (Powder Metallurgy Industry, Volume 21 Issue 5, October, 2011) written by L I Feng et al., that by adding Ho, the temperature stability of a material can be significantly improved, the intrinsic coercivity of the material is greatly improved, the remanence is reduced, the squareness of a J-H demagnetization curve is significantly improved, and crystal grains of the magnet are refined to a certain extent, allowing uniform distribution of a Nd-rich phase, and reduction in defects such as a cavities, making the magnet more compact.
It is described in “Effects of Adding Ho on Magnetic Performance and Temperature Stability of Sintered Nd—Fe—B Permanent Magnet Material” (Magnetic Material and Device, August, 2011) written by LIU Xianglian that by adding a suitable amount of Ho, formation of an a-Fe phase in a Nd—Fe—B alloy ingot is inhibited, and growth of Nd2Fe14B columnar crystals is promoted, allowing uniform distribution of a Nd-rich phase, and allowing the sintered Nd—Fe—B magnet to have a high degree of densification and a good microstructure; in addition, the intrinsic coercivity and the temperature stability of the magnet can be improved by adding a certain amount of Ho. Similar contents are described in “Effects of Adding Gd and Ho on Structure and Performance of Sintered Nd—Fe—B magnet” (Rare Earth, Volume 34 Issue 1, February 2013) written by ZHANG Shimao et al.
Based on the above, it can be concluded that, by adding Ho into the magnet, crystal grains of the magnet can be refined, allowing uniform distribution of a Nd-rich phase and improving the sintering capabilities of the magnet.
On the other hand, the method for manufacturing a Nd—Fe—B sintered magnet has been gradually improved. For example, a strip casting process (SC process) has been popularized in China since 2005, and the sintered magnet went into mass production with such a process in 2010. After raw materials are dissolved and casted with the SC process, it is easy to manufacture a thin-plate alloy, the crystallization structure in the thin-plate alloy is relatively uniform and fine, and the Nd-rich phase is distributed uniformly in micrometers. If the SC process is combined with a hydrogen decrepitation process, fine powder having an average grain size less than or equal to 10 μm can be obtained, and also, the sintering capabilities of the magnet can be significantly improved.
However, for the rare-earth magnet with a sharp improvement in sintering capabilities, if the inhibition of abnormal grain growth only relies on a small amount of impurities present in a grain boundary, the abnormal grain growth (AGG) would occur very easily.